This invention relates to a cleavage type safety valve provided in a metal vessel of, for example, a secondary cell. More specifically, the present invention relates to cleavage type safety valve for opening the pressure generated in the vessel in an emergency, for preventing deformation or the like of the vessel or for preventing destruction of equipment.
A high-performance secondary cell, such as lithium ion cell, which recently has come into widespread use, is characterized by a small size, lightness in weight and a large capacity. Lithium ion cells are used in increasing numbers in a portable equipment, such as a portable telephone set or a note-type computer.
However, this type of the high-performance secondary cell has a high energy density in the cell. While a combustible organic solvent is occasionally used as an electrolytic solution, such that, in case of an emergency, such as during shorting or overcharging in the cell, combustible gases tend to be evolved or ignited in the cell vessel and raise the internal pressure in the cell thus producing a problem such as inadvertent leakage of the cell contents.
If the internal pressure in the vessel exceeds the compressive strength, the vessel is deformed excessively. If the limit of compression is exceeded, the vessel explodes
If such accident occurs during use of the portable equipment, the user is endangered or the equipment is destroyed.
For avoiding this risk, a cleavage type safety valve which, if the internal pressure in the vessel is increased beyond a prescribed value, part of the vessel is cleaved to form an opening to open the internal pressure.
The cleavage type safety valve, used up to now, is classified into a marking type safety valve and a metal foil type safety valve, as selected depending on usage and application.
The marking type safety valve is of such a structure in which a marker punch having a tooth end with a wedge-shaped cross-section is driven into a portion of a vessel for machining a cleavage groove to a circular or the like contour so that, if the internal pressure of the vessel is increased to higher than a prescribed value, the thin-walled portion of the bottom of the cleavage groove is cleft to open the internal pressure.
The marker type safety valve has merits in facilitated machining and low manufacturing cost.
However, in driving the marker punch, cracks generated at the tooth end of the marker punch are left in the thin-walled portion after machining, such that, if the thin-walled portion is of a reduced wall thickness, cracks are formed extending through the thin-walled portion, thus possibly producing leakage.
The cleavage pressure in the marker type safety valve is determined by the pressure-receiving surface surrounded by the cleavage groove and the plate thickness of the thin-walled portion of the bottom of the cleavage groove. Thus, if it is desired to reduce the pressure-receiving surface for installment in a small-sized vessel and nevertheless it is desired to prevent the cleavage pressure from increasing, the plate thickness of the thin-walled portion needs to be decreased further. However, if the plate thickness of the thin-walled portion is further reduced, the ratio of rejection due to leakage is increased. Thus, difficulties are met in reducing the size of the marker type safety valve.
The metal foil type safety valve is of such a structure in which a metal foil is attached for closing the air vent hole in the vessel for hermetic sealing, so that, if the internal pressure in the vessel exceeds a prescribed value in case of an emergency, the metal foil becomes cleft to open the internal pressure.
In the conventional metal foil type safety valve, the metal foil is attached by a welding method. Since the lower limit value of the plate thickness of the metal foil that can be attached by the welding method is not more than one-half the lower limit value of the plate thickness of the thin-walled portion of the bottom of the cleavage bottom in case of the marking type cleavage cell, the metal foil type safety valve can, in this respect, be reduced more easily in size.
However, in attachment of the metal foil by the welding method, there are encountered problems of through-holes proper to the welding processing and technical difficulties in attachment and hermetic sealing by the welding method. In addition, equipment investment becomes costly thus presenting cost and quality problems.
The present invention has developed a metal foil type safety valve by the cold pressure welding method (referred to hereinafter as the present safety valve) in which a metal foil is attached by a cold pressure welding method for stopping air vent holes in a vessel to provide a safety valve.
The cold pressure welding method is a machining method in which portions of two metal components to be welded together are pressed and welded together at ambient temperature in a cold pressure welding metal mold.
The welding principle resides in inducing plastic deformation in the weld by pressure applied by the clad pressure welding metal mold, generating a new surface in both contact surfaces by friction between two contact surfaces and continuous pressing and holding in this state for inducing interatomic linkage for welding the two components together.
Heretofore, in cold pressure welding a metal foil with a plate thickness of not more than 0.3 mm, such as is used in a metal foil type safety valve, the metal foil tends to be cracked or fractured by the pressure applied by the cold pressure welding to produce leakage.
The present inventors have conducted re-check, of a number of operating conditions in cold pressure welding, including tooth profile of the cold pressure welding metal molds, weld shape, surface treatment or heat treatment in meeting with material types of the metal foils or the vessel, pressing rate for cold pressure welding and holding time at the lower dead point, by a trial-and-error method, and succeeded in attaching a metal foil with a plate thickness up to 0.01 mm by the cold pressure molding method, so far thought not possible, and in applying the attaching technique to the metal foil type safety valve.
The present safety valve has been confirmed to have many advantages, such as higher air tightness and a lesser number of occurrences of troubles in hermetic sealing, shortened machining time, and suppression of the machining time to approximately one-fifth, as compared to the case of the conventional metal foil type safety valve by the light beam welding method, as well as facilitated realization of an optimum cleavage pressure and diminished fluctuations in the cleavage pressure in case of mass production, which are two of the crucial factors as safety valves.
In an embodiment the present invention provides an improved cleavage type safety valve for a metal vessel that comprises a piece of metal foil connected to the vessel and over an air vent hole in the vessel by cold pressure welding the foil to the vessel. The cold pressure welding of the outer periphery of the foil to the vessel results in an hermetic seal over the vent hole provided by the metal foil. Further, the cold pressure welding of the foil to the vessel creates a thin-walled portion of the foil disposed radially inwardly of the weld. The thin-walled portion provides a reliable cleavage point in the event the vessel becomes over pressurized.
In an embodiment where the metal vessel has a plate thickness of about 0.8 mm, the metal foil layer has a plate thickness prior to the cold pressure welding of about 0.03 mm. However, the plate thickness at the thin-walled portion of the foil layer (that is disposed radially inwardly from the outer periphery of the foil layer that is cold pressure welded to the vessel) has a plate thickness of substantially less than 0.03 mm.
In an embodiment, the ratio of the plate thickness of the metal foil to the plate thickness of the vessel is about 3:80.
In an embodiment, the ratio of the plate thickness of the metal foil to the diameter of the air vent hole disposed in the vessel is about 3:150.
In an embodiment, the vessel and metal foil are fabricated from A3003.
In an embodiment, the vessel has dimensions of 34 mmxc3x976 mmxc3x9747 mm and a plate thickness of about 0.8 mm. The air vent hole has a diameter of about 1.5 mm and the plate thickness of the metal foil layer is about 0.03 mm. However, at the thin-walled portion inside of foil which is disposed inside of the outer periphery of the metal foil layer, which is cold pressure welded to the edge of the vessel which defines the air vent hole, the thickness of the metal foil layer is substantially less than 0.03 mm and can be as thin as 0.005 mm.
In an embodiment, the vessel further comprises a vessel body having an open end which is enclosed by a lid that is welded to the vessel body. The lid includes the air vent hole. The lid having dimensions of about 34 mmxc3x976 mm. The lid also having a plate thickness of 0.8 mm and further being fabricated from A3003.
In an embodiment, the vessel can withstand an internal pressure of at least 15 kg/cm2.
In an embodiment, the vessel can withstand an external pressure of at least 50 kg/cm2.
In an embodiment, the present invention provides a method of forming a cleavage type safety valve in a vessel which comprises the steps of providing a vessel body having an open end, providing a lid for the vessel body, forming an air vent hole in the lid, cold pressure welding an outer periphery of a metal foil layer over the air vent hole to hermetically seal the air vent hole and to provide a thin-walled section of the foil inside of the outer periphery or the weld, and welding the lid to the vessel body.
Other objects and advantages of the present invention will become apparent from reading the following detailed description and appended claims, and upon reference to the accompanying drawings.